After World War II, the US Air Force needed to know if pilots could eject from supersonic jets without facing certain death because of the shock of rapidly decelerating from the speed of sound to a near standstill. The transition exposed pilots to forces of over 40 or 50 Gs. (One G equals the force of gravity at the surface of the earth; 40 Gs is like a 7000-pound elephant falling on top of you.) Many doctors believed that 18 Gs was the most a human body could endure, but no one knew for sure. Flight surgeon John Paul Stapp volunteered to serve as the guinea pig in a series of physically brutal experiments to find out.

At Holloman Air Force Base in New Mexico, Stapp designed a rocket-powered sled that blasted down a 3500-foot track at speeds up to 750 mph before slamming into a pool of water that brought it to an abrupt halt. It went from 750 mph to zero in one second. Strong restraints made sure that the passenger didn’t continue their forward trajectory, though the restraints didn’t always work. One test dummy came free of the harness and was catapulted 700 feet through the air.

For his inaugural rocket sled ride, in 1947, Stapp went at a gentle 90 mph. The next day he advanced to 200 mph. And subsequently he kept signing up for more rides, upping his speed, probing the limits of human endurance. Over a period of seven years he rode the sled twenty-nine times.

Each time he rode the sled, the force of the deceleration hammered his body. He repeatedly endured blackouts, concussions, splitting headaches, cracked ribs, dislocated shoulders, and broken bones. One time, in a show of bravado, he set a broken wrist himself as he waited for medics to arrive. The greatest danger was to his eyes. Rapid deceleration causes the blood to pool with great force in the eyes, bursting capillaries and potentially tearing retinas. Even more disturbingly, when a human body comes to a stop that abruptly, there’s a real possibility the eyeballs will simply keep going — popping out of the skull and flying onwards.

An early version of the rocket sled

On Stapp’s final ride on 10 December 1954, this almost happened. Nine rockets propelled him to 632 mph, faster than a .45 calibre bullet. He outran a jet flying overhead. And when the sled hit the water, Stapp experienced a record-breaking 46.2 Gs of force.

Stapp survived, but he later wrote of the experience, “It felt as though my eyes were being pulled out of my head… I lifted my eyelids with my fingers, but I couldn’t see a thing.” He feared he’d permanently lost his vision, but thankfully his eyesight gradually returned over the next few days. However, on account of that final ride, he suffered vision problems for the rest of his life.

The Sensitive Testes

In 1933, either Herbert Woollard or Edward Carmichael had weights stacked on his testicles for the sake of science. It’s not possible to say exactly which one of these London-based doctors bore the unusual burden, because while both participated in the experiment, only one of them lay on a table and suffered the scrotal compression. The other one did the stacking. They never revealed who served in which capacity — nor how they chose who was to be the unlucky one.

Herbert Henry Woollard (left) and E.A. Carmichael

Their motive for this self-experiment was to better understand referred pain — the mysterious phenomenon in which injury to an internal organ causes pain to be felt elsewhere in the body. For instance, a heart attack may cause the sensation of pain in the arm. The two doctors noted that, of all the internal organs, the testicles were the most “accessible to investigation” and therefore seemed ideal for a study of referred pain.

During the experiment, the subject lay spread-eagled on a table, exposing his genitals. His colleague stooped over him and gripped the other man’s scrotal sac, drawing it forward and gently cradling it in his hand. He then rested a scale pan on a single testis, and carefully piled weights onto the pan, recording the reaction of the subject with each increase of weight.

One of Woollard and Carmichael's charts of testicular pain

Their results, which appeared in the journal Brain, were rather spare on colorful details. They described the agony of the victim only in dry, clinical details. For instance, they reported that 300 grams of weight produced slight discomfort in the right groin, while 650 grams caused severe pain on the right side of the body. However, they did confirm that injury to the testicles does cause pain to be referred throughout the body. For instance, as the weight on the testicle increased to over two pounds, the subject reported pain “of a sickening character” not only in his groin but also spreading across his back.

Woollard and Carmichael conducted a number of variations of the experiment, in which they numbed nerves leading to the testes in order to determine how this would alter the sensation. This produced the interesting finding that, even though they eventually numbed what they believed to be every nerve leading to the testes, they couldn’t entirely abolish the pain of compression. The testes are highly sensitive organs!

Their results remain the definitive word on this subject since no other scientists have ever repeated the experiment.